Platelet histidine-rich glycoprotein

Histidine-Rich Glycoprotein Stimulates Human Neutrophil Phagocytosis and Prolongs Survival through CLEC1A

Histidine-rich glycoprotein (HRG) is a multifunctional plasma protein and maintains the homeostasis of blood cells and vascular endothelial cells. In the current study, we demonstrate that HRG and recombinant HRG concentration dependently induced the phagocytic activity of isolated human neutrophils against fluorescence-labeled Escherichia coli and Staphylococcus aureus through the stimulation of CLEC1A receptors, maintaining their spherical round shape. The phagocytosis-inducing effects of HRG were inhibited by a specific anti-HRG Ab and enhanced by opsonization of bacteria with diluted serum. HRG and C5a prolonged the survival time of isolated human neutrophils, in association with a reduction in the spontaneous production of extracellular ROS. In contrast, HRG maintained the responsiveness of neutrophils to TNF-α, zymosan, and E. coli with regard to reactive oxygen species production. The blocking Ab for CLEC1A and recombinant CLEC1A-Fc fusion protein significantly inhibited the HRG-induced neutrophil rounding, phagocytic activity, and prolongation of survival time, suggesting the involvement of the CLEC1A receptor in the action of HRG on human neutrophils. These results as a whole indicated that HRG facilitated the clearance of E. coli and S. aureus by maintaining the neutrophil morphology and phagocytosis, contributing to the antiseptic effects of HRG in vivo.

Enhanced blood coagulation and fibrinolysis in mice lacking histidine-rich glycoprotein (HRG)

Histidine-rich glycoprotein (HRG) is a serum protein belonging to the cystatin superfamily. HRG may play a regulatory role in hemostasis and innate immunity. However, this role is uncertain because of a lack of rigorous testing in an animal model. We generated mice lacking the translation start point of exon 1 of the Hrg gene, effectively resulting in a null mutation (Hrg-/-). The mice were viable and fertile but had no HRG in their blood. Antithrombin activity in the plasma of Hrg-/- mice was higher than in the plasma of heterozygous Hrg+/- or wild-type Hrg+/+ mice. The prothrombin time was shorter in Hrg-/- mice than in Hrg+/- and Hrg+/+ mice. Bleeding time after tail tip amputation in Hrg-/- mice was shorter than in Hrg+/+ mice. The spontaneous fibrinolytic activity in clotted blood of Hrg-/- mice was higher than in Hrg+/+ mice. These findings suggest that HRG plays a role as both an anticoagulant and an antifibrinolytic modifier, and may regulate platelet function in vivo.

Histidine‐rich glycoprotein: A novel adaptor protein in plasma that modulates the immune, vascular and coagulation systems

Histidine-rich glycoprotein (HRG) is an abundant plasma glycoprotein that has a multidomain structure, interacts with many ligands, and has been shown to regulate a number of important biological processes. HRG ligands include Zn(2+) and haem, tropomyosin, heparin and heparan sulphate, plasminogen, plasmin, fibrinogen, thrombospondin, IgG, FcgammaR and complement. In many cases, the histidine-rich region of the molecule enhances ligand binding following interaction with Zn(2+) or exposure to low pH, conditions associated with sites of tissue injury or tumour growth. The multidomain nature of HRG indicates that it can act as an extracellular adaptor protein, bringing together disparate ligands, particularly on cell surfaces. HRG binds to most cells primarily via heparan sulphate proteoglycans, binding which is also potentiated by elevated free Zn(2+) levels and low pH. Recent reports have shown that HRG can modulate angiogenesis and additional studies have shown that it may regulate other physiological processes such as cell adhesion and migration, fibrinolysis and coagulation, complement activation, immune complex clearance and phagocytosis of apoptotic cells. This review outlines the molecular, structural, biological and clinical properties of HRG as well as describing the role of HRG in various physiological processes.